Joining member for Z-interconnect in electronic devices without conductive paste

ABSTRACT

A layer for use in a modular assemblage for supporting ICs is formed with metal contacts for assembly by making a sandwich of metal interconnect members between two layers of dielectric; drilling holes through the dielectric, stopping on a metal layer bonded to the bottom surface of the module; forming blind holes stopping on the interconnect members; and plating metal through the volume of the via, both full and blind holes, thereby forming vertical and horizontal connections in a layer that be stacked to form complex interconnect assemblies.

TECHNICAL FIELD

[0001] The field of the invention is that of packaging integrated circuits, in particular assembling layers of dielectric surrounding metal interconnections that support and connect integrated circuits.

BACKGROUND OF THE INVENTION

[0002] In assembling composite sets of interconnections surrounded by insulators, the art has used a “Z-connection” method in which layers of conducting materials are connected using an electrically conductive paste.

[0003] Typically, as shown in FIGS. 4A and 4B, a set of dielectric sheets 460 and 465 having metal connections embedded in them have a set of vertical vias and horizontal metal lines or sheets. The horizontal lines or sheets 10 are placed between two sheets of dielectric 15 that are bonded in a sandwich. (Layer 10 is optional.) Holes or apertures between the metal sheets provide vertical connections. Fixed metal vias 420 formed by plating are used together with vias 436 formed by forcing a conductive paste made of epoxy or other plastic filled with a metal powder into a hole. Top and bottom layers 410 are made of copper or other metal. The metal is etched to define contact pads at locations 402 and 404 on which solder bumps are placed The assemblage is bonded together, by a lamination process performed at elevated temperatures and pressures such that the dialectic materials stick to each other and to the metal. The assemblage in FIG. 4A is generic and the one in FIG. 4B is customized. On the right of FIG. 4B, denoted with dotted line 442, a set of vertical members make a vertical path between upper and lower contacts 402 and 404. On the left in FIG. 4B, denoted by a dotted line 444, a conductive paste via 436 extends downwardly from a solder ball in the upper left of the Figure. A half-via 420′ connects via 436 to a horizontal line 10, at the end of which a second half-via 420′ makes contact with the next via 436. Two more vias carry the path down to the lower solder ball.

[0004] Those skilled in the art will appreciate that use of conductive paste gives rise to various problems: the paste is messy—it squeezes out of holes and can cause short circuits unless carefully cleaned up.

[0005] The paste is thick and often does not fill a hole properly, especially blind holes, causing an open circuit or higher resistivity than specified.

[0006] Paste particle size can be a problem in filling small holes. Pastes containing silver are expensive.

[0007] These problems are addressed by the present invention.

SUMMARY OF THE INVENTION

[0008] The invention relates to a set of modules for forming IC packaging in which the vertical members are formed by plating.

[0009] A feature of the invention is drilling holes through a dielectric sheet, stopping on a layer metal layer and thereby forming a hole penetrating through the dielectric and having a closed bottom.

[0010] A feature of the invention is plating a contact layer of a second metal at the bottom of the hole before plating a first metal through the bulk of the hole.

[0011] Another feature of the invention is filling through-holes halfway, then filling blind holes that reach down only to horizontal conductors embedded in the dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1A through 1G illustrate steps in forming a dielectric sheet according to the invention.

[0013]FIG. 2 illustrates a composite via according to the invention.

[0014]FIGS. 3A through 3D illustrate alternative embodiments of the invention.

[0015]FIGS. 4A and 4B illustrate a prior art module using layers with conductive paste.

DETAILED DESCRIPTION

[0016] Referring to FIG. 1 there is shown a series of steps in making an interconnect structure used in connecting integrated circuits. The starting point is a dielectric sandwich of two copper layers 20 on the top and bottom of two dielectric layers 15, e.g. polymer printed circuit board material. When finished, the layer will be one module that will be assembled with others to form the interconnect structure. At the center, horizontal metal members 10 will carry signals into and out of the plane of the paper and left to right in the plane of the paper.

[0017] At the next stage, shown in FIG. 1B, upper layer 20 has been removed, e.g. by etching, and vertical holes 30 have been formed in spaces between members 10. Illustratively, the holes are drilled by a laser, though other methods are acceptable. Preferably, the laser drilling is adjusted in power so it stops on the top surface of bottom layer 20, thereby forming a bottom cap in the hole and a starting surface for the plating steps that will follow.

[0018] The hole will be filled with copper (referred to in the following claims as the first metal), which is inexpensive and has a high conductivity, but first a bottom cap 22 of Sn (the second metal) is plated on the top of copper bottom layer 20. Sn melts at a lower temperature and therefore solders better to adjacent layers. Typically, a thickness of several microns of Sn is adequate to get the benefit of its superior properties. In an alternative embodiment, described below, there could be two layers of thin metal layers in the bottom of the via 30, referred to as the second and third metals.

[0019] The holes 30 are filled by plating copper. In this example, the first plating step only fills the holes 30 to about half their height (denoted 35 in FIG. 1C). A second set of blind holes 32 are formed, as shown in FIG. 1D, stopping on some of the horizontal members 10. These blind holes will be used for horizontal connections. The holes, both holes 30 and 32, are filled with copper, shown as 36 and 36′. The second set of shallow blind vias are optional. If not present, plating of copper in the deeper vias can proceed in a single step. Excess copper will be formed on the top surface of dielectric 15 as part of typical manufacturing practice, shown in FIG. 1E as bumps 38. The excess copper can be removed with chemical-mechanical polishing, leaving a coplanar surface of dielectric and metal.

[0020] A corresponding method can be performed on the bottom of the layer, leaving a set of vertical vias that extend along an axis from top to bottom and a set of offset vias (or offset vertical interconnects) that start at one location on the top surface and exit from a location on the bottom surface that is offset from the top location.

[0021] A second layer 42 of Sn, (the fourth metal) may then be plated on the top of the copper (shown in FIG. 1F). Advantageously, a mask is not required for this step, since the Sn self-aligns to the copper. The Sn 42 is shown as projecting above the surface 17 of the dielectric, but a slight etching step to recess the copper could be performed if precise planarity is required.

[0022] Last, as shown in FIG. 1G, bottom layer 20 of copper is removed, exposing bottom surface 19 of the dielectric layer and the Sn at the base of the vias.

[0023] In a subsequent bonding step, Sn contacts that are aligned with a contact vertically above or below will fuse with those other metal contacts to establish continuous electrical connections through surface 17 and a corresponding surface 19 of the next layer. Some of the Sn could be adjacent to a layer of a different material and different structure. Typically, the thickness of the layer that forms caps 42 is about several microns, which is not an obstacle in mechanical alignment or in having the top surface 17 shown in good mechanical contact with an adjacent lower surface 19 of the composite layer above it.

[0024] Referring now to FIG. 2, there is shown in cross section a single via with a more elaborate structure, before removal of lower layer 20. Copper 36 has been deposited after deposition of a second metal 21, illustratively Au at a bottom cap thickness of 100 to 2000 Angstroms, and a third metal, Sn, with an intermediate thickness of several microns. The via has a composite bottom cap of the second and third metals. At the top, a layer of fourth metal (Sn) has been plated with a top cap thickness of several microns and a fifth metal (Au) has been plated with a top cover thickness of 100 to 2000 Angstroms. The top has a composite top cap with the fourth and fifth metals.

[0025] Referring now to FIG. 3, there is shown an alternative embodiment, in which FIG. 3A shows a layer with vias 30, open at the bottom. The vias 30 are filled with copper using a process similar to that shown in FIG. 1. The drilled vias stop at the top surface of bottom copper layer 20 in FIG. 3A, which is removed by etching. Excess plated copper on the top or bottom surface is removed by chemical-mechanical polishing.

[0026] A blanket layer of copper 120 is deposited on the top surface and then a layer of photoresist 145 is deposited and patterned to leave openings above the vias. Sn 142 and Au 144 are plated on to copper 120, in the opening above the vias. This self-aligned plating step eliminates the need for patterning and etching the metal layers. The result is shown in FIG. 3D, with a set of vertical vias having a composite pad on the top and bottom with an attachment surface of Au. Blind vias could be formed to make contact with horizontal members 10, as was described with respect to FIG. 1.

[0027] In this case, the first metal is copper, the second and fourth metals are Sn and the third and fifth metals are Au. In other cases, the identity of the metals may change.

[0028] As an alternative to the use of Cu to fill the vias in FIG. 3B, vias can be drilled through the entire thickness of FIG. 3A, including top and bottom layers of Cu, 20. The vias are filled with an electrically conductive paste. A layer of copper 120 is deposited, e.g. by bonding a sheet of copper foil to the top and bottom of the structure. A layer of photoresist 145 is deposited and patterned to leave openings above the vias. Sn 142 and Au 144 are plated on to copper 120, in the opening above the vias. This self-aligned plating step eliminates the need for patterning and etching the metal layers. The result is shown in FIG. 3D, with a set of vertical vias having a composite pad on the top and bottom with an attachment surface of Au. Blind vias could be formed to make contact with horizontal members 10, as was described with respect to FIG. 1. In this case, the first metal is copper, the second and fourth metals are Sn and the third and fifth metals are Au. In other cases, the identity of the metals may change.

[0029] In making up a package for ICs, a set of layers is assembled, having the vias and horizontal connections called for in the design. The layers are assembled and bonded together, e.g. by reflowing solder contacts on facing surfaces. Conductive layers are formed on top and bottom surfaces, as in FIG. 4. Solder bumps may be formed on contacts that are fabricated by patterning the conductive layers. One or more ICs are attached to the set of contacts on the top surface and the bottom contacts are attached to a suitable substrate, e.g. a printed circuit board.

[0030] While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims. 

What is claimed is:
 1. A method of forming a module for assembling an interconnect structure comprising the steps of: providing at least one sheet of dielectric, at least a bottom surface of said dielectric being covered with a conductive bottom layer; forming a first set of vias from a top dielectric surface of said dielectric through at least one of said of spaces to a top surface of said bottom layer; plating said first set of vias with a first metal up to a first level, to form a vertical member of said first metal; and removing excess amounts of said first metal outside said set of vias, to form a top via surface coplanar with said top dielectric surface.
 2. A method according to claim 1, further comprising the steps of: after said step of forming a set of vias, plating a second metal on said top surface of said bottom layer with a bottom cap thickness of said second metal, wherein said vertical member has a bottom cap of said second metal.
 3. A method according to claim 2, further comprising the steps of: after said step of plating said second metal, plating a third metal on a top surface of said bottom cap with an intermediate cap thickness of said third metal, wherein said vertical member has a composite bottom cap of said second and third metals.
 4. A method according to claim 2, further comprising the steps of: after said step of removing excess amounts of said first metal outside said set of vias, plating a fourth metal on said top surface of said first metal with a top cap thickness of said fourth metal, wherein said vertical member has a bottom cap of said second metal and a top cap of said fourth metal.
 5. A method according to claim 1, further comprising the steps of: laminating a set of horizontal metal members, separated by spaces in a horizontal plane between at least two sheets of dielectric, at least a bottom surface of said dielectric being covered with said conductive bottom layer; forming a first set of vias from a top dielectric surface of said dielectric through to a top surface of said conductive bottom layer; forming a second set of vias from said top dielectric surface of said dielectric through to a top surface of at least some of said horizontal metal members; forming a third set of vias from a bottom dielectric surface of said dielectric through to a bottom surface of at least some of said horizontal metal members; plating said first, second and third set of vias with said first metal up to respective top and bottom dielectric surfaces; and removing excess amounts of said first metal outside said first, second and third sets of vias, to form a top via surface and a bottom via surface coplanar with said top and bottom dielectric surfaces and having a set of top and bottom contacts at least some of which are connected by a set of vertical interconnects and at least some of which are connected by a set of offset interconnects having top and bottom contacts displaced laterally.
 6. A method according to claim 5, further comprising the steps of: after said step of forming said sets of vias, plating a second metal on said top surface of said bottom layer with a bottom cap thickness of said second metal, wherein said vertical member has a bottom cap of said second metal.
 7. A method according to claim 6, further comprising the steps of: after said step of plating said second metal, plating a third metal on a top surface of said bottom cap with an intermediate cap thickness of said third metal, wherein said vertical member has a composite bottom cap of said second and third metals.
 8. A method according to claim 6, further comprising the steps of: after said step of removing excess amounts of said first metal outside said set of vias, plating a fourth metal on said top surface of said first metal in said first, second and third sets of vias with a top cap thickness of said fourth metal, wherein said vertical members of said first set of vias have a bottom cap of said third metal and said vertical members and said offset interconnects of said first, second and third sets of vias a top cap of said fourth metal.
 9. A method of forming a module for assembling an interconnect structure comprising the steps of: providing at least one sheet of dielectric, at least a bottom surface of said dielectric being covered with a conductive bottom layer; forming a set of vias from a top dielectric surface of said dielectric through at least one of said of spaces to a top surface of said bottom layer; filling said set of vias with a first conductive material up to a first level, to form a vertical member of said first conductive material; providing a layer of a first metal on at least one surface of said sheet of dielectric; patterning a layer of photosensitive material to expose a set of terminal portions of said first metal adjacent to said set of vias; and plating said set of terminal portions with a second metal, wherein said vias are provided with a set of self-aligned contact layers.
 10. A method according to claim 9, further comprising the steps of: providing a layer of a first metal on a second surface of said sheet of dielectric opposite said at least one surface; patterning a second layer of photosensitive material on said second surface to expose a second set of terminal portions of said first metal adjacent to said set of vias; and plating said second set of terminal portions with said second metal, wherein said vias are provided with a set of self-aligned contact layers on both top and bottom surfaces.
 11. A method according to claim 9, further comprising the steps of: after said step of plating said sets of terminal portions with said second metal, plating said sets of terminal portions with a third metal, wherein said sets of terminal portions have a composite contact comprising a layer of said first metal, said second metal and said third metal.
 12. A method according to claim 9, in which said conductive material is applied as a paste.
 13. A method of forming a module for assembling an interconnect structure according to claim 10, further comprising the steps of: laminating a set of horizontal metal members, separated by spaces in a horizontal plane between at least two sheets of dielectric, at least a bottom surface of said dielectric being covered with a bottom cap; forming a first set of vias from a top dielectric surface of said dielectric through to a top surface of said bottom cap; plating said set of vias with a first metal up to a first level; forming a second set of vias from said top dielectric surface of said dielectric through to a top surface of at least some of said horizontal metal members; forming a third set of vias from a bottom dielectric surface of said dielectric through to a bottom surface of at least some of said horizontal metal members; plating said first, second and third set of vias with said first metal up to respective top and bottom dielectric surfaces; and removing excess amounts of said first metal outside said set of vias, to form a top via surface and a bottom via surface coplanar with said top and bottom dielectric surfaces and having a set of top and bottom contacts at least some of which are connected by a set of vertical interconnects and at least some of which are connected by a set of offset interconnects having top and bottom contacts displaced laterally.
 14. A method according to claim 13, in which said conductive material is applied as a paste.
 15. A method of forming an interconnect structure for connecting at least one integrated circuit comprising the steps of forming a set of N interconnect modules by: (a) providing at least one sheet of dielectric, at least a bottom surface of said dielectric being covered with a conductive bottom layer; (b) forming a first set of vertical vias from a top dielectric surface of said dielectric through at least one of said of spaces to a top surface of said bottom layer; (c) plating said set of vias with a first metal up to a first level, to form a vertical member of said first metal; and (d) removing excess amounts of said first metal outside said set of vias, to form a top via surface coplanar with said top dielectric surface; and further: repeating said steps (a) through (d) to assemble said set of N interconnect modules, individual members of said set having patterns of horizontal interconnect members and vias to establish a designed set of interconnections; bonding said set of N interconnect modules, to establish a module top surface; and forming a set of contact pads adapted for making contact to at least one integrated circuit on said module top surface, said set of contact pads being connected to said set of interconnections formed in said set of N interconnect modules.
 16. A method according to claim 15, further comprising the steps of: after said step of forming a set of vias, plating a second metal on said top surface of said bottom layer with a bottom cap thickness of said second metal, wherein said vertical member has a bottom cap of said second metal.
 17. A method according to claim 16, further comprising the steps of: after said step of removing excess amounts of said first metal outside said set of vias, plating a fourth metal on said top surface of said first metal with a top cap thickness of said fourth metal, wherein said vertical member has a bottom cap of said second metal and a top cap of said fourth metal.
 18. A method according to claim 15, further comprising the steps of: laminating a set of horizontal metal members, separated by spaces in a horizontal plane between at least two sheets of dielectric, forming a set of offset vias that start from a first location on said top dielectric surface, extend horizontally through one of said horizontal metal members and exit vertically from said horizontal metal member through said bottom surface of said dielectric;
 19. A method according to claim 18, further comprising the steps of: after said step of forming said sets of vias, plating a second metal on said top surface of said bottom layer with a bottom cap thickness of said second metal, wherein said vertical member has a bottom cap of said second metal.
 20. A method according to claim 19, further comprising the steps of: after said step of removing excess amounts of said first metal outside said set of vias, plating a fourth metal on said top surface of said first metal in said first, second and third sets of vias with a top cap thickness of said fourth metal, wherein said vertical members of said first set of vias have a bottom cap of said third metal and said vertical members and said offset interconnects of said first, second and third sets of vias a top cap of said fourth metal. 